Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	$ 2,096,852	$ 2,061,467
Allowance for Credit Losses	(375)	(397)	$ (878)	$ 0
Gross Unrealized Gains	94,430	138,110
Gross Unrealized Losses	(15,038)	(2,554)
Fair Value	2,175,869	2,196,626
U.S. government
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	161,956	170,110
Gross Unrealized Gains	9,539	13,504
Gross Unrealized Losses	(1,073)	(257)
Fair Value	170,422	183,357
U.S. Agency
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	28,883	28,902
Gross Unrealized Gains	2,783	3,970
Fair Value	31,666	32,872
Non-U.S. government & agency
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	8,296	10,298
Gross Unrealized Gains	407	667
Gross Unrealized Losses	(148)
Fair Value	8,555	10,965
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	358,224	384,015
Gross Unrealized Gains	14,007	18,789
Gross Unrealized Losses	(3,019)	(733)
Fair Value	369,212	402,071
ABS/CMBS/MBS
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	215,177	213,223
Allowance for Credit Losses		(17)
Gross Unrealized Gains	3,713	5,580
Gross Unrealized Losses	(658)	(413)
Fair Value	218,232	218,373
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	796,042	753,404
Allowance for Credit Losses	(375)	(380)
Gross Unrealized Gains	42,200	64,501
Gross Unrealized Losses	(4,664)	(933)
Fair Value	833,203	816,592
Municipal
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	528,274	501,515
Gross Unrealized Gains	21,781	31,099
Gross Unrealized Losses	(5,476)	(218)
Fair Value	$ 544,579	$ 532,396